EP2595930B1 - Glass ceramic as a cooktop for induction heating having improved colored display capability and heat shielding and method for producing such a cooktop - Google Patents

Description

The invention relates to a glass ceramic as a cooking surface for induction heating with improved colored display capability and heat shielding consisting of a transparent, colored glass ceramic plate with high quartz mixed crystals as the predominant crystal phase, and to a process for their production and their use. Transparent, colored glass ceramic cooktops are black when viewed from above and thus prevent the technical installations under the cooktop from being seen through.

Hobs with a glass ceramic plate as the cooking surface are the current state of the art. Such glass ceramic plates are usually in the form of flat plates or are three-dimensionally deformed.

Depending on the type of heating, there are different types of glass ceramic cooktops. Radiation-heated cooking surfaces are those in which radiant heaters are made to glow by electrical energy due to their ohmic resistance. The most widespread band radiators have maximum values of the emitted radiation in the wavelength range from approximately 1600 to 3000 nm. Alternatively, the radiant heaters can also consist of halogen radiators. These heat up faster and emit radiation at shorter wavelengths with maximum radiation values in the range from about 1000 to 1600 nm.

In order to achieve quick heating-up times, most of the heat radiation should be let through to the bottom of the pot through the glass ceramic plate and absorbed there. Therefore higher transmission values in the infrared at 1600 nm of more than 45% are desired. Common glass ceramic cooktops that are established in the market are around 75%.
With induction cooktops, the energy is coupled directly into the metal pan base via electromagnetic waves generated in coils and absorbed there via eddy currents. This direct heating enables short heating times to be achieved. The glass ceramic plate is now heated via the heated pan base and transfers this heat inwards to the internals under the glass ceramic plate.
With gas-heated glass ceramic cooktops, the glass ceramic plates usually contain holes into which the gas burners are integrated. Depending on the type of flame and the proximity of the flame to the edge of the holes, there are different thermal loads. In the case of very strong and low-lying gas burners, these are so high that it is necessary to use a temperature-stable glass ceramic instead of a glass. Even with gas-heated cooking surfaces, the built-in components under commercially available glass ceramic plates can heat up considerably due to the heat radiation emitted by the gas flame.
Especially for induction and gas-heated glass ceramic cooktops, the high infrared transmission of commercially available glass ceramic plates is therefore rather a disadvantage.
The improved heat shielding is important for special operating conditions when using cooking surfaces. With induction cooktops with a booster function, higher power is temporarily provided at the beginning of the cooking process in order to shorten cooking times. If it comes to high temperatures because this is not recognized in time by the control electronics and the power is reduced, the pan base heats up excessively. Another operating condition with excessive The bottom of the pan may heat up when the pan boils empty and heats up due to the lack of food. In such rare operating conditions, but which must be tolerated by the glass ceramic cooktop, the bottom of the pot can heat up to around 500 ° C. This releases increased heat radiation. In such operating conditions, there is briefly increased heat radiation, which stresses the technical installations under the glass ceramic plate.
The technical structure of induction hobs under the glass ceramic plate consists of coils that generate the inductive field. The wires of these coils are provided with plastic insulation. Other sensitive installations are, for example, electro-optical sensors which are used to measure the temperature of the pan base. There are also contact elements for temperature measurement that are also encapsulated with organic materials. The electrical feeds as well as light emitting diodes (LED) and color displays are encapsulated with organic materials and very sensitive. Often, thermally insulating materials such as, for example, induction cooktops between the technical fittings and the glass ceramic plate. B. mica-containing plates and foils arranged for heat shielding. The installation and the special design of these shields are associated with higher costs because, among other things, areas for display windows or optical sensors have to be left out, while on the other hand reliable heat shielding of all critical areas must be guaranteed. Overall, it is therefore desirable to provide the glass ceramic plate itself with a higher heat shield.

The required temperature difference resistance is lower for inductive or gas-heated glass ceramic hobs than for radiation-heated hobs. While temperature difference strengths of more than 700 ° C are required for the safe use of radiation-heated cooking surfaces, this is partly included for values below 600 ° C for inductive or gas-heated cooking surfaces and for systems with complex electronic control Values of less than 400 ° C. Since the temperature difference resistance is also largely determined by the thermal expansion, the glass ceramic for induction or gas-heated cooking surfaces can have higher values of the thermal expansion. While common specifications for cooking surfaces for radiation-heated cooking surfaces are approx. 0 ± 0.15.10 ^-6 / K, this value measured between room temperature and 700 ° C for inductive and gas-heated glass ceramic cooking surfaces can be up to 0 ± 2.10 ^-6 / K. This opens up a wider range of compositions for the display of glass ceramics. It is also advantageous that the thermal expansion is better related to other materials such. B. can adjust the inorganic colors with which cooking surfaces are decorated. This means less tension in the decorated areas and thus increased strength of the glass ceramic plates.

The improved colored display capability represents a further essential requirement for new cooking surfaces. To improve the ease of use and for safe operation, modern glass ceramic cooking surfaces are equipped with colored displays such as 7-segment displays or operating displays, which are installed below the glass ceramic plate.
Colored displays give the user information about the switched-on state of the individual cooking zones, the control setting and also whether the cooking zones are still hot after being switched off. Light-emitting diodes are used as usual colored displays. These colored displays are particularly important for induction hobs. In contrast to the radiation-heated cooking surfaces, where the heated cooking zone can be recognized by its red-hot color, the induction cooking surfaces are black and visually unchanged even when in use when heated. It is therefore desirable to identify the operating status and residual heat with special colors or display functions.

Due to the coloring and the associated transmission curves, conventional glass ceramic cooktops have a limited selection of possible colored LED displays. By default, the displays that can be used are red or, if necessary, orange, other displays such as, in particular, blue or white have so far been hardly possible. The usual red light-emitting diodes shine at wavelengths around 630 nm and the transmission of the glass ceramic cooktop is set to about 2 to 12% for this wave.
Especially for the use of available blue LEDs, it would be desirable to achieve a transmission of greater than 0.4% in the range from 380 to 500 nm with at least one wavelength.
The improved colored display capability enables manufacturers of cooking appliances to differentiate their products with a glass ceramic cooktop based on the design. Due to the type and design of the colored displays under the glass ceramic plate, a manufacturer or brand-typical design can take place. The design over light with the new options, which is then possible due to the transmission characteristics of the glass ceramic, can set unmistakable accents for the brand on the market.

The glass ceramic cooktop itself should appear black when viewed from above and have an aesthetic appearance. In order to prevent the disruptive view of the technical components under the glass ceramic cooktop and to avoid the glare caused by radiating radiators, in particular bright halogen radiators, the light transmission of the glass ceramic cooktop is limited. Therefore, the light transmission for the human eye must not be higher than about 2.5%, because otherwise the aesthetic black view will be lost and the technical installations under the glass ceramic plate will be visible under normal lighting conditions. In order to meet the requirements for the display capability, the light transmission is at least 0.5%.
The transmission values apply regardless of the thickness of the glass ceramic plate because they are decisive for the function of the cooking surface. Glass ceramics with high quartz mixed crystals as the predominant crystal phase are made from crystallizable lithium aluminum silicate glasses.

The large-scale manufacture of these glass ceramics takes place in several stages. First, the crystallizable starting glass is melted from a mixture of cullet and powdery raw materials at temperatures usually between 1500 and 1650 ° C. Arsenic and / or antimony oxide is typically used as a refining agent in the melt. These refining agents are compatible with the required glass ceramic properties and lead to good bubble qualities in the melt. Even if these materials are firmly integrated in the glass structure, they are disadvantageous from a safety and environmental point of view. For example, special precautionary measures must be taken when extracting and processing raw materials and because they evaporate from the melt.

After melting and refining, the glass is usually subjected to hot shaping by rolling or, more recently, floating to produce plates. A low melting temperature and a low processing temperature V _{A are} desired for economical production. Furthermore, the glass must not show any devitrification during the shaping. This means that no disruptive crystals may be formed during the shaping, which impair the strength in the original glasses and the glass ceramics made from them. Since the shaping takes place near the processing temperature V _A (viscosity 10 ⁴ dPas) of the glass, it must be ensured that the upper devitrification temperature of the melt is below the processing temperature in order to avoid the formation of disruptive crystals. In a subsequent temperature process, the starting glass is transferred into the glass-ceramic article by controlled crystallization. This ceramization takes place in a two-stage temperature process, in which initially by nucleation at a temperature between 680 and 810 ° C., usually from ZrO ₂ / TiO ₂ mixed crystals. SnO ₂ can also be involved in nucleation. When the temperature then increases, the high-quartz mixed crystals grow on these seeds. High crystal growth rates, as desired for economical, fast ceramization, are achieved at temperatures of 820 to 970 ° C. At this maximum manufacturing temperature, the structure of the glass ceramic is homogenized and the optical, physical and chemical properties are adjusted. If desired, the high quartz mixed crystals can then be converted into keatite mixed crystals. The conversion to keatite mixed crystals takes place when the temperature rises in a temperature range of approx. 970 to 1250 ° C. Glass ceramics with keatite mixed crystals have higher values of thermal expansion.
The conversion is also associated with crystal growth to average crystallite sizes of 100 nm and above, and associated light scattering. Glass ceramics with keatite mixed crystals as the main crystal phase are therefore no longer transparent but translucent or opaque. When used as a cooking surface, the light scatter has a negative effect on the display capability, since the displays under the glass ceramic plate are no longer clearly recognizable and an annoying halo is formed.

The use of SnO ₂ as a safe refining agent has recently been described for the production of environmentally friendly glass ceramic cooktops. In order to achieve good bubble qualities, at conventional melting temperatures (maximum approx. 1680 ° C), halide compounds are preferably used as additional refining agents in addition to SnO ₂ . So is in the Japanese applications JP 11 100 229 A and JP 11 100 230 A the use of 0.1-2% by weight of SnO ₂ and 0-1% by weight of Cl is described. According to these documents, the coloring is achieved by adding V ₂ O ₅ as the sole colorant.

This achieves high transmission values in the infrared because the V ₂ O _{5 has} the special property of absorbing in the range of visible light and being transparent in the infrared. This is essential for radiation-heated cooking surfaces, but has the disadvantages described for heat shielding for induction and gas-heated cooking surfaces.

The use of SnO ₂ in connection with high-temperature refining above 1700 ° C to achieve good bubble qualities is in the DE 199 39 787 C2 described. However, this document does not provide any indication of the achievement of good heat shielding. An essential feature of the font is an infrared transmission of greater than 65% at 1600 nm as it is essential for radiation-heated cooking surfaces.

An earlier type of glass ceramic cooktop, known under the name Ceran Color®, manufactured by SCHOTT AG, had good color display capabilities in blue and red. Ceran Color® is colored by the addition of NiO, CoO, Fe ₂ O ₃ and MnO and refined by Sb ₂ O ₃ . This combination of color oxides results in a light transmission of typically 1.2% for cooking surfaces with a usual thickness of 4 mm. The transmission in the range from 380 nm to 500 nm is 0.1 - 2.8% depending on the wavelength. At a wavelength of 630 nm, which is common for red light-emitting diodes, the transmission is approx. 6%. The IR transmission at 1600 nm is less than 20%. The transmission curve of Ceran Color® is in the book " Low Thermal Expansion Glass Ceramics ", editor Hans Bach, Springer-Verlag Berlin Heidelberg 1995, shown on page 66 (ISBN 3-540-58598-2 ). The composition is in the book " Glass-Ceamic Technology ", Wolfram Höland and George Beall, The American Ceramic Society 2002 listed in Tables 2-7. In terms of an environmentally friendly cooking surface, the use of the refining agent Sb ₂ O _{3 is} disadvantageous. Due to the low transmission in the range of green light of <0.1% at 580 nm, the neutral display with white LED is not available in addition to green.

In the DE 102008050263 A1 discloses a transparent, colored cooking surface with improved colored display capability. On the basis of this document, a new type of glass ceramic cooktop was recently launched on the market under the brand name CERAN Hightrans Eco, manufactured by SCHOTT AG. The cooktop consists of a glass ceramic with high-quartz mixed crystals as the predominant crystal phase and does not contain any of the chemical refining agents arsenic oxide and / or antimony oxide except for inevitable traces. The improved color display capability is characterized by transmission values of greater than 0.1% in the visible light range, greater than 450 nm in the entire wavelength range, light transmission in the visible range of 0.8 to 2.5% and transmission in the infrared at 1600 nm from 45 to 85%. Due to the high transmission in the infrared, this glass ceramic is excellently suitable for radiation-heated cooking surfaces. However, the high infrared transmission is disadvantageous for use as an induction or gas-heated cooking surface. For the development of glass ceramic plates that are to be used specifically for use as induction or gas-heated cooking surfaces, it is therefore desirable to lower the infrared transmission to values below 45% and preferably below 40% in order to improve the heat shield.

The EP 1465460 A2 discloses a glass ceramic cooktop which, measured in the CIE color system, has a Y value (brightness) of 2.5-15 with standard light C and a thickness of 3 mm. The terms "brightness" and light transmission correspond to the same measurement. The Y value is identical to the value of the light transmission, measured according to DIN 5033. With this light transmission, improved displays for blue and green light-emitting diodes are achieved. The compositions disclosed are refined with As ₂ O ₃ and / or Sb ₂ O ₃ , sometimes in combination with SnO ₂ . The coloring is done by V ₂ O ₅ . The comparative example shows that with a light transmission of 1.9%, the display capability for blue and green LEDs with the listed material compositions is insufficient. However, the claimed high values of light transmission of at least 2.5% and preferably higher are disadvantageous with regard to the coverage of the electronic components below the cooking surface. In addition, the aesthetic black appearance of the cooking surface is impaired when viewed from above.

US 2005/252503 A1 describes translucent or opaque hobs with different shades when viewed from above.

EP 2 226 303 A2 describes a process for melting and refining a glass melt for a starting glass of a LAS glass ceramic.

WO 2010/137000 A2 and WO 2011/089220 A1 describe transparent colored glass ceramics with different transmission characteristics.

It is an object of the invention to provide cooking surfaces for induction heating with improved colored display capability and heat shielding, and a method for their production, the cooking surfaces consisting of a glass ceramic with high quartz mixed crystals as the predominant crystal phase, except for inevitable traces of none of the chemical refining agents arsenic oxide and / or contain antimony oxide and are suitable for economical and environmentally friendly production. For economical production, the starting glasses should be easy to melt and refine, have a high devitrification stability and be ceramizable in short times. The cooking surfaces according to the invention are intended to meet all the other requirements placed on cooking surfaces, such as, for example: chemical resistance, temperature resistance and high temperature / time resistance with regard to changes in their properties (such as thermal expansion, transmission, build-up of voltages).

These objects are achieved by a cooking surface according to claim 1 and by a method according to claim 3.

The cooktops have transmission values of greater than 0.4% at at least one wavelength in the blue between 380 and 500 nm, a transmission of> 2% at 630 nm, a transmission of less than 45% at 1600 nm and a light transmission in the visible range of less than 2.5%.

The transmission of greater than 0.4% at at least one wavelength in the blue between 380 and 500 nm enables good display capability with available blue displays. Since these displays, which mostly consist of light-emitting diodes, shine at a specific wavelength with a typical width of 15 nm, it is sufficient if the transmission is matched to this wavelength by more than 0.4%. Such wavelengths for the radiation of standard blue LED displays are, for example, at 430 and 470 nm. The coordination of the transmission of the glass ceramic to the wavelengths at which the blue LED used radiate also has the advantage that the light transmission in the cooking surface is not increased too much. If the transmission is increased uniformly in the wavelength range from 380 to 500 nm, the light transmission according to the invention of up to 2.5% will otherwise be left very quickly. By combining several color oxides, the transmission curve can be increased especially for discrete wavelength ranges. This means that an increase in the display capability takes place on a more significant scale than can be achieved with a reduction in the material thickness. With regard to an improved display capability, this is also of great relevance for future energy-efficient (less powerful) displays. It can already be seen today that the wavelength-selective increases in transmission realized in this way offer the potential, too with lower performance of the LED displays or displays to improve the display capability.
In the area of ultraviolet light of less than 350 nm, the cooking surface has the low transmission values of less than 0.01%, as is standard for glass ceramic cooking surfaces. The blocking of the UV light is advantageous for protecting the organic components, such as. B. glue in the technical fittings under the cooking surface, as well as protection when cooking, if blue LED displays with ultraviolet radiation are used for the display.

The light transmission of less than 2.5% according to the invention ensures that the disruptive view of the technical components under the glass ceramic cooktop is prevented and the aesthetic black appearance is ensured under supervision. Due to the transmission of> 2% at 630 nm, common red light-emitting diode displays are clearly visible.
Due to the set infrared transmission of less than 45%, measured at 1600 nm, the requirements for improved heat shielding are met. In the case of induction or gas-heated glass ceramic cooktops, the electrical and electronic installations under the glass ceramic plate are better protected against the heat radiation from the hot pan base. This is particularly important in operating conditions with excessive heating of the pan base. This also reduces the effort required to protect the internals by means of thermally insulating materials by providing the glass ceramic plate itself with a higher heat shield.
The infrared transmission is preferably reduced to values of less than 40% because the heat shielding improves further.

Since the transmission and light transmission values according to the invention are decisive for the function of the cooking surface, they apply regardless of the thickness of the glass ceramic plate, which is between 2.5 and 6 mm. Smaller thicknesses are disadvantageous in terms of strength and greater thicknesses are uneconomical because they require more material and reduce the rate of ceramization. The thickness of the cooking surface is usually around 4 mm. Since the thickness and the concentration of the colorants are equally important factors in the extinction, the person skilled in the art will easily adjust the required transmission of the cooking surface to the respective thickness via the concentration of the colorants. If the cooking surface is produced using rollers, the underside is usually provided with knobs to protect it from injuries that reduce strength during production. Often, the underside of the cooktop is smoothed in the area of the colored displays with transparent organic polymer in order to avoid optical distortion caused by the knobs. In the case of cooktops with a smooth underside without knobs, colored displays are undistorted and brighter.

The cooking surfaces according to the invention have a composition without the refining agents arsenic and antimony oxide and are therefore technically free from these components which are disadvantageous from the point of view of safety and environmental protection. As components of the raw materials, these components are usually present in contents of less than 0.05% by weight.

In order to ensure the demands on the temperature difference resistance for induction or gas-heated cooking surfaces of up to 600 ° C., the glass ceramic cooking surfaces according to the invention have
Coefficient of thermal expansion of up to (0 ± 2) • 10 ^-6 / K.

The process according to the invention for producing a cooking surface for induction heating with improved colored display capability and heat shielding is distinguished by the fact that it forms a transparent, colored glass ceramic with high-quartz mixed crystal as the predominant crystal phase and that, except for inevitable traces, on the chemical refining arsenic and / or antimony oxide is dispensed with and the cooking surface has transmission values of greater than 0.4% at at least one wavelength in the blue between 380 and 500 nm, light transmission in the visible range of less than 2.5%, transmission of> 2% at 630 nm and a transmission in the infrared at 1600 nm of less than 45%, preferably less than 40%.

The transmission of the cooking surface is greater than 500%, a transmission from> 2 to less than 12% at 630 nm, a transmission in the near infrared at 950 nm of values of greater than 0.1% in the range of visible light in the entire wavelength range greater than 30% and a light transmission in the visible range of 0.5 - 2%.

With these values, the color display capability is further improved and the different requirements for the transmission process are further optimized. Due to the transmission of greater than 0.1% in the range of visible light in the entire wavelength range of greater than 500 nm, not only the blue but also differently colored displays such as green, yellow or orange are clearly recognizable. Displays with white light are less distorted in color by this transmission process. Limiting the transmission at 630 nm to values of less than 12% prevents the red LED displays from changing, that is to say they appear too bright. The transmission in the near infrared at 950 nm of greater than 30% ensures that the usual operating sensors that function on an optical basis can be used. A further improved coverage of the technical installations beneath the cooktop glass ceramic and a particularly aesthetic black appearance in reflected light is achieved if the light transmission is less than 2%. The display capability is further improved if the light transmission from the cooking surface is at least 0.5%.

The cooking surface according to the invention contains the main components of the glass ceramic composition (in% by weight on an oxide basis): Li ₂ O 1.5 - 4.2 ∑ Na ₂ O + K ₂ O 0.2 - 1.5 MgO 0-3 ∑ CaO + SrO + BaO 0 - 4 ZnO 0-3 B ₂ O ₃ 0-2 Al ₂ O ₃ 19-23 SiO ₂ 60-69 TiO ₂ 1.5-6 ZrO ₂ 0.5 - 2 P ₂ O ₅ 0-3 SnO ₂ 0.1 - <0.6 ∑ TiO ₂ + ZrO ₂ + SnO ₂ 3.8 - 6 and Fe ₂ O ₃ 0.03-0.3 in combination with CoO 0.05-0.3, NiO 0.05-0.3, manganese compounds> 0 - 2, V ₂ O ₅ 0-0.06 and Cr ₂ O ₃ 0-0.03.

The oxides Li ₂ O, Al ₂ O ₃ and SiO ₂ within the specified limits are necessary components of the high quartz mixed crystals. A minimum Li ₂ O content of 1.5% by weight is required for easily controllable crystallization. Higher contents of more than 4.2% by weight often lead to unwanted devitrification in the manufacturing process.

In order to avoid higher viscosities of the starting glass and the undesired devitrification of mullite during shaping, the Al ₂ O ₃ content is preferably limited to a maximum of 23% by weight. The SiO ₂ content should not exceed 69% by weight because this component greatly increases the viscosity of the glass. For good melting of the glasses and for low shaping temperatures, higher levels of Al ₂ O ₃ and SiO _{2 are} uneconomical. The minimum SiO ₂ content should be 60% by weight because this is advantageous for the required cooking surface properties, such as chemical resistance and temperature resistance.

MgO, ZnO and P ₂ O _{5 can be built} into the high-quartz mixed crystals as further components. The ZnO content is limited to a maximum of 3% by weight due to the problem of the formation of undesired crystal phases such as zinc spinel (gahnite) during the ceramization. The MgO content is limited to a maximum of 3% by weight, because otherwise it would inadmissibly increase the expansion coefficient of the glass ceramic. The addition of P ₂ O ₅ is beneficial for melting and shaping the starting glass.

The addition of the alkalis Na ₂ O, K ₂ O and the alkaline earths CaO, SrO, BaO and B ₂ O ₃ improve the meltability and the devitrification stability in the shaping of the glass. However, the contents have to be limited because these components are not built into the crystal phases, but essentially remain in the residual glass phase of the glass ceramic. Excessively high contents impair the crystallization behavior during the conversion of the crystallizable starting glass into the glass ceramic, here in particular at the expense of faster ceramization speeds. In addition, higher levels have an unfavorable effect on the time / temperature resistance of the glass ceramic. The sum of the alkalis Na ₂ O + K ₂ O should be at least 0.2% by weight and a maximum of 1.5% by weight.
The sum of the alkaline earths CaO + SrO + BaO should not exceed 4% by weight. In addition to the residual glass phase between the crystals, the alkalis and alkaline earths mentioned also accumulate on the surface of the glass ceramic. At the Ceramization forms an approx. 200 to 1000 nm thick glassy surface layer that is almost free of crystals and that is enriched in these elements and depleted in lithium. This glassy surface layer has a favorable effect on the chemical resistance of the glass ceramic.

The minimum amount of the nucleating agents TiO ₂ , ZrO ₂ and SnO ₂ is 3.8% by weight. During ceramization, they form high-density crystal nuclei during nucleation, which serve as a base for the growth of the high-quartz mixed crystals during crystallization. The high seed density leads to a high crystal density with average crystallite sizes of less than 100 nm, which avoids light scattering which is disruptive for the displays. The nucleating agent contents are correlated with the nucleation rate and are therefore important for shorter ceramization times. Levels higher than the total of 6% by weight impair the devitrification stability. For improved devitrification stability, the SnO ₂ content is limited to less than 0.6% by weight. Higher contents lead to the crystallization of Sn-containing crystal phases on the contact materials (eg Pt / Rh) during the shaping and must be avoided at all costs. The ZrO ₂ content is up
2% by weight, since higher contents impair the melting behavior of the batch during glass production and the devitrification stability during the shaping can be impaired by the formation of crystals containing ZrO ₂ . The minimum content of ZrO ₂ should be 0.5% by weight in order to promote a high ceramization rate. The TiO ₂ content is between 1.5 and 6% by weight. The minimum amount should not be undercut, so that high nucleation rates for achieving high
Ceramization speeds are ensured. The content should not exceed 6% by weight, because otherwise the devitrification stability will deteriorate.

A combination of at least two color oxides is required to set the transmission according to the invention with improved colored display capability and heat shielding. Fe ₂ O ₃ contents from 0.03 to 0.3

% By weight are combined with color oxides from the group V ₂ O ₅ , CoO, Cr ₂ O ₃ , and NiO. In addition to the color oxide Fe ₂ O ₃ , the glass ceramic contains NiO and CoO in minimum contents of 0.05% by weight in order to set the infrared transmission at 1600 nm to values of less than 45%.
The combination of the color oxides makes it possible to make do with smaller amounts of the expensive coloring agent V ₂ O _{5, which is} classified as a hazardous substance. The content of the other color oxides is preferably at least twice as high as that of the V ₂ O ₅ .
With the color oxide contents according to the invention, it is possible to achieve all requirements for the transmission process, such as specification-compliant light transmission, reduced infrared transmission, and display capability for standard red light-emitting diodes together with the desired improved display capability for blue and differently colored light displays.

Other coloring components such as manganese, copper, selenium, rare earth, molybdenum compounds can be used to support the coloring and to lower the transmission in the infrared. Their content is limited to a maximum of about 1% by weight because these compounds generally reduce the transmission in the blue. Manganese compounds are contained in higher contents up to about 2% by weight because of the weaker coloring effect.
By adding 50 - 400 ppm Nd ₂ O ₃ it is possible to mark the glass ceramic cooking surface. The absorption band of the Nd in the near infrared at 806 nm is in a range of high transmission values of the glass ceramic and is thus distinctive in the transmission spectrum. As a result, the cooking surface material can be safely assigned to the manufacturer and good recycling is possible with optical body detection methods.

The water content of the starting glasses for the production of the cooking surfaces according to the invention depends on the choice of raw materials and the Process conditions in the melt usually between 0.015 and 0.06 mol / l. This corresponds to β-OH values of 0.16 to 0.64 mm ^-1 for the crystallizable starting glasses.

Good meltability and rapid ceramizability of the starting glass are desired for economical production. It is necessary to increase the nucleation and ceramization rate by an appropriately selected composition. It has proven to be advantageous here to increase the contents of the nucleating agents TiO ₂ + ZrO ₂ + SnO ₂ in order to increase the nucleation rate, while the P ₂ O ₅ content must be selected at lower values.
In order to improve the meltability, it has proven advantageous to reduce the contents of SiO ₂ , ZrO ₂ and Al ₂ O ₃ and the proportion of the components which form the residual glass phase of the glass ceramic, such as the alkalis Na ₂ O and K ₂ O as well as increasing the alkaline earths CaO, SrO, BaO. The upper limits of the contents of ZnO and MgO are preferably lowered in order to improve the devitrification stability during the shaping.
The SnO ₂ content is preferably 0.1 to 0.5% by weight. A minimum content of 0.1% by weight is required for the refining of the glass. Furthermore, the SnO ₂ acts as a nucleating agent and if V ₂ O _{5 is also used} as a colorant, it is required as a reox partner so that the vanadium ion can be reduced to the coloring, lower oxidation state when ceramized. The SnO ₂ content is a maximum of 0.5% by weight for improved devitrification stability during shaping.

The term "essentially consists of" means that the listed components should be at least 96%, generally 98% of the total composition. A large number of elements such as F, Cl, the alkalis Rb, Cs or elements such as Hf are common impurities in the batch raw materials used on an industrial scale. Other Compounds such as those of the elements Ge, rare earths, Bi, W, Nb, Ta, Y can be added in small proportions.

In order to improve the bubble quality, in addition to the SnO _{2 used} , other refining additives such as CeO ₂ , MnO ₂ , sulfate, sulfide, and halide compounds can also be used. Their contents are usually limited to amounts up to 2% by weight. Good bubble qualities in the production of cooking surfaces are those with bubble numbers of less than 5, preferably less than 3 bubbles / kg of glass (based on bubble sizes greater than 0.1 mm).

As another important result of the addition of Fe ₂ O ₃ , it was found that this significantly supports the refining. In combination with the SnO ₂ as the main refining agent, the Fe ₂ O ₃ also releases oxygen and is thereby reduced to the Fe ²⁺ . In order that the addition of Fe ₂ O ₃ as an additional refining agent in combination with SnO _{2 has a} particularly advantageous effect, the content in a particularly preferred form should be at least 0.05% by weight.

Both SnO ₂ and Fe ₂ O ₃ are high-temperature refining agents and release the oxygen required for refining in sufficient quantities at high melting temperatures from around 1650 ° C. The turnover that is decisive for the refining effect increases with the temperature of the melt. A temperature treatment of the melt of more than 1700 ° C and more than 1750 ° C thus delivers further improved results with regard to the bubble quality. For improved bubble numbers of less than 3 bubbles / kg of glass with economically favorable higher throughputs, high-temperature purification with temperatures of the glass melt of greater than 1700 ° C., preferably greater than 1750 ° C., is advantageous.

Rapid ceramicization means a thermal treatment for crystallizing the glass ceramic with a duration of less than 2 hours, preferably less than 80 minutes.

In the process for ceramization according to the invention, the thermally relaxed crystallizable starting glass is heated within 1 to 30 minutes to the temperature range of the transformation temperature Tg of the glass of approximately 680 ° C. The high heating rates required can be realized on an industrial scale in roller furnaces. Above this temperature up to about 810 ° C is the area with high nucleation rates. The temperature range for nucleation is traversed over a period of 8 to 30 minutes. The temperature of the glass containing nuclei is then raised to a temperature of 820 to 970 ° C. in the course of 2 to 30 minutes, which is characterized by high crystal growth rates of the high-quartz mixed crystal phase. This maximum temperature is maintained for up to 30 minutes. The structure of the glass ceramic is homogenized and the optical, physical and chemical properties are adjusted. The glass ceramic obtained is cooled to 800 ° C. with cooling rates of approx. 2 to 40 ° C./min and then quickly to room temperature.

By adding reducing agents in powder and / or liquid form to the starting mixture, the coloring effect of the V ₂ O _{5 can be} enhanced or blue-coloring Ti ³⁺ can be produced. Metals, carbon and / or oxidizable carbon or metal compounds such as Al or Si powder, sugar, charcoal, SiC, TiC, MgS, ZnS are suitable for this. Gaseous reducing agents, such as forming gas, are also suitable. The reducing agents mentioned are suitable for lowering the pO _{2 of} the melt. Since vanadium oxide is an expensive raw material, it is economically advantageous to minimize the content.

Preferably, one or more differently colored displays, such as blue, green, yellow, orange or white, are arranged under the cooking surface according to the invention with improved colored display capability instead of or in addition to the usual red displays. The colored displays consist of light-emitting electronic components, mostly of light-emitting diodes. All forms of advertisements are possible, both point and area. It can too black and white and colored displays or screens are shown with significantly improved color fidelity. In addition to displaying operating states, it is also possible for the user to work interactively on the cooking surface. It can e.g. B. Read recipes, view pictures or communicate with the intranet. It can be controlled via touch-sensitive screens. The underside of the cooking surface can have the usual knobs or be made smooth. The improved display capability is particularly noticeable on cooktops with a smooth underside, as the colored displays are undistorted and brighter. The cooking surface can contain areas of reduced thickness for the displays. Since the transmission depends exponentially on the layer thickness, the brightness of the display, e.g. B. on a screen, greatly increased. However, the other areas of the cooking surface should be thicker so that they have the light transmission according to the invention.

The cooking surface can not only be shaped as a flat plate, but also three-dimensionally shaped, e.g. folded, angled or curved plates can be used. The plates can be rectangular or in other shapes, as well as three-dimensionally deformed areas such as e.g. Bars or woks included.

The following examples further illustrate the present invention.

For some exemplary embodiments, compositions of the crystallizable starting glasses and the refining conditions are listed in Table 1. The glasses 1 to 4 are glasses according to the invention and the glass 5 is a comparison glass which leads to a glass ceramic outside of the present invention.
Due to typical impurities in the large-scale batch raw materials used, the compositions do not add up to 100 % By weight. Typical impurities, even if not intentionally introduced into the composition, are F, Cl, B, P, Rb, Cs, Hf, which are usually less than 0.1% by weight. They are often introduced via the raw materials for the chemically related components, such as Rb and Cs via the Na or K raw materials, or Sr via the Ba raw material and vice versa.

The starting glasses of Table 1 were melted from raw materials customary in the glass industry at temperatures of approximately 1620 ° C. for 4 hours. After the mixture had melted in crucibles made of sintered silica glass, the melts were poured into Pt / Rh crucibles with internal crucibles made of silica glass and homogenized by stirring at temperatures of 1550 ° C. for 30 minutes. After this homogenization, the glasses were refined at 1640 ° C. for 2 hours. Pieces of size 140 × 80 × 30 mm ^{3 were then} cast and, in order to avoid stresses, cooled in a cooling oven, starting at a temperature of 660 ° C. to room temperature. The test samples for the measurements were prepared from the castings.
Glass No. 4 with the same composition as Glass No. 3 was instead refined at 1850 ° C. for 1 hour. The positive influence of the high-temperature purification with regard to the bubble quality is visually evident in this laboratory melt.

1. a) schnelles Aufheizen von Raumtemperatur auf 680°C innerhalb von etwa 10 min,
2. b) Temperaturerhöhung von 680 °C auf 730 °C mit einer Heizrate von 10 °C/min, weiteres Aufheizen mit 5,2 °C/min auf 810 °C,
3. c) Temperaturerhöhung von 810°C auf Maximaltemperatur T_max von 920°C mit einer Heizrate von 6 °C/min, Haltezeit t_max von 6 min bei Maximaltemperatur,
4. d) Abkühlen von Maximaltemperatur auf 800°C mit 5,5 °C/min, dann schnelle Abkühlung auf Raumtemperatur.

The ceramization of the starting glasses was carried out with the following temperature / time program:

1. a) rapid heating from room temperature to 680 ° C within about 10 min,
2. b) temperature increase from 680 ° C to 730 ° C with a heating rate of 10 ° C / min, further heating at 5.2 ° C / min to 810 ° C,
3. c) temperature increase from 810 ° C to maximum temperature T _max of 920 ° C with a heating rate of 6 ° C / min, holding time t _max of 6 min at maximum temperature,
4. d) cooling from maximum temperature to 800 ° C at 5.5 ° C / min, then rapid cooling to room temperature.

Table 2 shows the properties in the ceramic state, such as the thermal expansion between 20 and 700 ° C and the transmission values for selected wavelengths. The values were determined on polished plates with the specified thicknesses typical for cooking surfaces. The optical measurements were made with standard light C, 2 degrees.

• Fig. 1a zeigt "Ordinate spektraler Transmissionsgrad = 1.0 (Transmission = 100%),
• Fig. 1b zeigt "Ordinate spektraler Transmissionsgrad = 0.1 (Transmission = 10%)".

The 1a and 1b show transmission spectra of the glass ceramic according to the invention according to Example No. 4 from Table 2 in different axis resolutions.

• Fig. 1a shows "ordinate spectral transmittance = 1.0 (transmission = 100%),
• Fig. 1b shows "ordinate spectral transmittance = 0.1 (transmission = 10%)".

Glass No. 3 fulfills the desired requirements for display capability and heat shielding for cooktops of thicknesses 3.5 (example 3) and 4 mm (example 4) and is characterized by a particularly advantageous combination of color oxides. The high-temperature refining of glass No. 4 with the same composition as glass No. 3 intensifies the coloring effect, in particular of the vanadium oxide, in the visible light range. This is shown by the direct comparison of Example 5 with Example 3 since both have a thickness of 3.5 mm. The curves of the transmission spectra of the glass ceramics according to the invention Example Nos. 3 and 5 from Table 2 "ordinate spectral transmittance = 0.1 (transmission = 10%)" are in Fig. 2 shown.

In the comparative glass ceramic example 6, the requirements for good display capability and improved heat shielding cannot be achieved with the thicknesses customary for cooking surfaces.

Due to their content of high quartz mixed crystal as the predominant crystal phase, the examples have the desired low values of thermal expansion, measured in the temperature range between 20 and 700 ° C. The values characteristic of the invention for the transmission at the different wavelengths and for the light transmission, which is synonymous with "brightness" Y, are listed in the table. Table 1: Compositions and refining temperatures of crystallizable starting glasses according to the invention and comparison glass 5. Glass no. 1 2nd 3rd 4th 5 Compositions in% by weight based on oxide like glass No. 3 Li ₂ O 3.82 3.87 3.86 2.98 Na ₂ O 0.59 0.59 0.57 0.46 K ₂ O 0.26 0.26 0.26 0.20 MgO 0.31 0.31 0.32 1.47 CaO 0.41 0.42 0.40 0.31 SrO - - - 0.52 BaO 2.31 2.32 2.29 1.86 ZnO 1.44 1.49 1.44 1.55 Al ₂ O ₃ 20.4 20.6 20.6 20.0 SiO ₂ 64.8 65.1 65.0 65.3 TiO ₂ 3.09 2.54 3.11 4.44 ZrO ₂ 1.34 1.35 1.37 0.28 SnO ₂ 0.25 0.25 0.25 0.22 P ₂ O ₅ - - 0.10 - MnO ₂ 0.20 0.20 0.024 - CoO 0.21 0.21 0.089 0.047 Fe ₂ O ₃ 0.18 0.18 0.077 0.095 NiO 0.27 0.27 0.13 0.15 V ₂ O ₅ 0.015 - 0.018 0.012 Refining temperature, refining time ° C h 1640 2 1640 2 1640 2 1850 1 1640 2 Example No. 1 2nd 3rd 4th 5 6 Glass no. 1 2nd 3rd 3rd 4th 5 Thermal expansion α _20/700 10 ^-6 / K 0.43 0.31 0.15 0.15 0.15 1.18 Transmission standard light C, 2 ° thickness mm 2.5 3.0 3.5 4.0 3.5 2.5 400 nm % 0.3 0.2 0.9 0.4 0.3 <0.01 450 nm % 1.3 3.8 1.8 1.0 0.8 0.2 470 nm % 0.9 2.1 1.1 0.6 0.5 0.4 500 nm % 0.3 0.2 0.3 0.1 0.1 0.5 600 nm % 0.6 0.8 1.6 0.9 0.9 1.4 630 nm % 4.5 7.8 6.9 4.8 4.2 3.3 700 nm % 31.7 49.9 30.4 26.0 22.5 11.2 950 nm % 45.5 40.3 55.3 51.5 53.6 50.8 1600 nm % 20.7 14.1 33.4 28.9 33.0 52.9 Light transmission Y % 0.9 1.3 1.3 0.8 0.8 1.1

Claims (4)

1. Glass-ceramic cooking surface for induction heating having improved coloured display capability and heat shielding, consisting of a transparent, coloured glass-ceramic plate which has a thickness in the range from 2.5 mm to 6 mm and has high-quartz mixed crystals as predominant crystal phase, where the glass-ceramic contains none of the chemical refining agents arsenic oxide and antimony oxide except for unavoidable traces in an amount of less than 0.05%,
   characterized by transmission values of the cooking surface of:

   > 0.4% at at least one wavelength in the blue range from 380 to 500 nm,

   > 2% - < 12% at 630 nm,

   < 45% at 1600 nm, preferably < 40%,

   and a light transmission in the visible range of 0.5-2%,

   > 0.1% in the range of visible light in the total wavelength range > 500 nm,

   in the near infrared at 950 nm > 30%,

   and in that

   the composition of the glass-ceramic (in percent by weight on an oxide basis) contains, as main constituents: Li₂O 1.5 - 4.2 ∑ Na₂O+K₂O 0.2 - 1.5 MgO 0 - 3 ∑ CaO+SrO+BaO 0 - 4 ZnO 0 - 3 B₂O₃ 0 - 2 Al₂O₃ 19 - 23 SiO₂ 60 - 69 TiO₂ 1.5 - 6 ZrO₂ 0.5 - 2 P₂O₅ 0 - 3 SnO₂ 0.1 - < 0.6 ∑ TiO₂+ZrO₂+SnO₂ 3.8 - 6 and Fe₂O₃ 0.03 - 0.3 in combination with CoO 0.05 - 0.3 NiO 0.05 - 0.3 Manganese compounds > 0 - 2 V₂O₅ 0 - 0.06 Cr₂O₃ 0 - 0.3.
2. Glass-ceramic cooking surface for induction heating according to Claim 1,
   characterized in that
   the glass-ceramic has a bubble count of less than 3 bubbles/kg as a result of a temperature of the glass melt of greater than 1700°C, preferably greater than 1750°C.
3. Process for producing a glass-ceramic cooking surface for induction heating with fast ceramicization of the crystallizable starting glass according to Claim 1 or 2 over a total time of less than 2 hours, preferably less than 80 minutes,
   characterized in that
   the ceramicization is carried out using the following program:

   a) increasing the temperature of the crystallizable glass to the temperature range of about 680°C within a period of 1-30 minutes;

   b) increasing the temperature of the crystallizable glass within the temperature range of nucleation from 680 to 810°C over a period of from 8 to 30 minutes;

   c) increasing the temperature of the glass containing crystallization nuclei within a period of from 2 to 30 minutes to the temperature range of a high crystal growth rate from 820 to 970°C;

   d) holding within the temperature range at the maximum temperature of from 820 to 970°C for up to 30 minutes in order to allow crystals of the type high-quartz mixed crystals to grow on the crystallization nuclei and then

   e) rapid cooling of the glass-ceramic obtained to room temperature.
4. Process for producing a glass-ceramic cooking surface for induction heating according to Claim 3, characterized in that
   a bubble count of less than 3 bubbles/kg of glass is achieved as a result of a temperature of the glass melt of greater than 1700°C, preferably greater than 1750°C.

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